I ask this question is in concern that the story of Abel (هابیل) and Cain (قابیل) when the first murder on earth happened between mankind. In that case, one side didn't fight back the murder but simply warn the other side of the hell fire.
However; I don't understand why we should not allow self defense even against Muslims.
For example, if a Muslim is shooting at you or going to stab you, are one suppose to just let the other side do it? Or any type of self defensive actions are allowed in Islam in this type of scenario?
Please state authentic Qur'an n hadith reference. Jazakallahir!